The newspaper said the numbers leaving the party could be even higher, as Labour’s systems record active resignations in real time but lapsed memberships appear in the data only after six months.

The leaked data showed total Labour membership at 528,180, down from a peak of 554,000 in July, but still far above the 200,000 members it reportedly had in May 2015. Official figures are only published in the party’s annual report.

Commenting on the reports of the decline in membership, the shadow chancellor, John McDonnell, said: “These figures are mostly seasonal or the result of the lapsing of members who joined last summer and were unable to vote in the leadership election. But Labour is now the largest party in western Europe. And that is because people have joined Labour in record numbers under Jeremy Corbyn’s leadership, which is something other parties can only dream of matching.”

However, former PLP chairman Lord Watts told the Times: “I think the tide is turning. I imagine people are losing heart because they can see the polls, they’re talking to their neighbours and people they work with, and are coming to the conclusion Labour is not doing well and, at this point, not convincing the public.”

Reports of a membership slump come after figures showed Labour raised less money through donations than the Liberal Democrats in the last three months of last year. Party sources said it was the first time the Lib Dems had outstripped Labour in fundraising over a quarter, taking in donations totalling £1,972,904 compared with £1,970,055 for Corbyn’s party.

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A £1m gift for the Lib Dems came from Gregory Nasmyth, a former journalist who made a fortune from last year’s £950m sale of his family’s Argus Media oil data company.

According to Electoral Commission records it is the first time Nasmyth has made a donation as an individual to a political party.

Labour said it is a “mass membership party, proud to be funded by members and working people”. Figures for the fourth quarter of 2016 – the first full quarter of Theresa May’s premiership – showed the Conservatives taking the lion’s share of donations, with £3,610,983.